"The other animals here are OK - they have lemurs, elephants and kangaroos as neighbours, and they show no sign of panic."

Emmen lies in the north-eastern Netherlands, near the German border.

Mr Landman said he had consulted a French baboon expert who had witnessed such baboon hysteria in the wild, triggered by awareness of a predator. But the French expert said such hysteria had not lasted as long as in the Emmen case.

"The first deviation we saw was on Monday evening," Mr Landman said.

"We were going to bring them to the night enclosure - it normally takes a minute for all 112 to enter, but it took more than an hour to get them all inside. Then the next morning it was a problem to get them out, and then they were immediately sitting in the trees and on the rocks doing nothing at all."

He said some males in the hierarchy must have got frightened for some reason, and the rest of the baboons followed their lead.

According to Mr Landman, it is unlikely they would have been spooked by a fox, as the zoo is in the city centre, and they are used to seeing herons flying low over their enclosure, so a bird of prey is also an unlikely cause.

The Emmen baboons had similar scares in 1994, 1997 and 2007 and some would have experienced the previous hysteria, as hamadryas baboons can reach the age of 30, Mr Landman said.